1. Field of the Invention
The present invention relates to an ultrasonic testing method and an ultrasonic testing equipment for detecting a flaw existing on a tubular test object such as a steel pipe or tube (hereinafter referred to as “pipe” when deemed appropriate) using an ultrasonic wave. Particularly, the present invention relates to an ultrasonic testing method and an ultrasonic testing equipment capable of evaluating the position of a tilted flaw in the thickness direction of a tubular test object and the tilt angle of the tilted flaw easily when flaws having various tilt angles (tilted flaws) with respect to an axial direction of a tubular test object are detected manually, and also relates to an ultrasonic testing equipment capable of obtaining a high reliability test result without any change in the posture of a ultrasonic probe with respect to the tubular test object when the flaws are detected manually.
2. Description of the Related Art
As demand for higher quality pipes grows in recent years, there is an increasing trend that nondestructive test standards for the pipes are becoming more stringent.
For example, a seamless pipe, which is a typical pipe, is manufactured by punching a billet with a piercer to form a hollow shell and rolling the hollow shell with a mandrel mill or the like. The seamless pipe has flaws having various tilt angles (hereinafter referred to as “tilted flaws” when deemed appropriate) with respect to the axial direction.
A tilted flaw is believed to be caused by deformation in the axial direction of a longitudinal crack originally existing on the billet in the above manufacturing process or transfer of a flaw existing on a guide face of a guide shoe for maintaining a path center of a hollow shell. Therefore, the tilt angle of the tilted flaw with respect to the axial direction of the seamless pipe changes depending on a difference in a pipe diameter of the seamless pipe or a cause for occurrence thereof. That is, there are tilted flaws with various tilt angles on the seamless pipe.
Since there is a trend of tighter service conditions of the seamless pipes from year to year, higher quality is demanded and accurate detection of the above tilted flaws is also sternly demanded.
Conventionally, various methods for detecting the tilted flaws existing on the seamless pipes have been proposed.
In Patent Literature 1 (Japanese Unexamined Patent Publication No. 55-116251), for example, a method for detecting a tilted flaw by arranging an ultrasonic probe at an appropriate position and tilt angle depending on the position and tilt angle of the tilted flaw to be detected is proposed.
However, the method described in Patent Literature 1 has a problem that extremely much time and manpower are needed because the tilt angle of the ultrasonic probe must be changed each time in accordance with the tilt angle of the tilted flaw to be detected. Also, to detect tilted flaws with various tilt angles existing on the seamless pipe in one round of flaw-detecting work, as described above, many ultrasonic probes must be provided, each of which is arranged with a different tilt angle. That is, there are problems that large equipment is required and soaring costs are entailed, in addition to complicated arrangements/settings and calibration of ultrasonic probes.
To solve the problems of the method described in the above Patent Literature 1, a flaw detecting method that applies an ultrasonic probe array in which a plurality of transducers (elements for transmitting/receiving ultrasonic waves) are arranged in a single row is proposed in Patent Literature 2 (Japanese Unexamined Patent Publication No. 61-223553). More specifically, transversal ultrasonic waves are propagated within the pipe by aligning an arrangement direction of the transducers with the axial direction of the pipe and arranging the ultrasonic probe decentralized from an axial center of the pipe. Then, according to this method, the tilted flaws with the various tilt angles are detected by changing the tilt angle (tilt angle with respect to the axial direction of the pipe) of ultrasonic waves transmitted and received by the ultrasonic probe using electronic scanning that electrically controls transmission/reception timing of the ultrasonic wave by each transducer.
However, two main problems (first problem and second problem) shown below exist in the method described in Patent Literature 2.
<First Problem>
According to the method described in Patent Literature 2, the intensities of echoes from tilted flaws are different even if they are the tilted flaws of the same size, depending on the tilt angles of the tiled flaws. The reason is that even if the tilt angle of ultrasonic wave is changed by electronic scanning corresponding to the tilt angle of each tilted flaw such that the extension direction of the tilted flaw and a propagation direction (propagation direction viewed from a normal direction of a tangential plane of the pipe including an incident point of the ultrasonic wave) of the ultrasonic wave transmitted by the ultrasonic probe are orthogonal to each other, an external refraction angle (incident angle to an external surface flaw existing on the external surface of the pipe) and an internal refraction angle (incident angle to an internal surface flaw existing on the internal surface of the pipe) are changed corresponding to the tilt angle of each tilted flaw (corresponding to the propagation direction of the ultrasonic wave). If the intensities of the echoes from the tilted flaws are different depending on the tilt angle of the tilted flaw, there is a possibility that the detection of a harmful flaw may be prevented or minute flaws that need not to be detected may be over-detected.
<Second Problem>
If electronic scanning for electrically controlling transmission/reception timing of the ultrasonic wave by each transducer of an ultrasonic probe array described in Patent Literature 2 is used to change the tilt angle of the ultrasonic wave transmitted and received by the ultrasonic probe, electronic scanning must be repeated as many times as required depending on the tilt angle of the tilted flaw to be detected in a specific area of the pipe. That is, for example, to detect three tilted flaws with different tilt angles, electronic scanning must be repeated three times in the specific area of the pipe, and flaw-detection efficiency is reduced to ⅓ when compared with detection of flaws with a unidirectional tilt angle. As described above, the method described in Patent Literature 2 has the problem that the flaw-detection efficiency goes down as the number of the tilt angles of the tilted flaws to be detected increases.
In Patent Literature 3 (Japanese Unexamined Patent Publication No. 59-163563), on the other hand, a method for causing the ultrasonic wave to enter in any direction using a group of transducers arranged in a matrix state in order to detect the tilted flaws with the various tilt angles is proposed. More concretely, an incident direction of the ultrasonic wave is arbitrarily changed by selecting an appropriate number of arbitrary transducers from the group of transducers and by performing electronic scanning for electrically controlling transmission/reception timing (driving time) thereof. Then, it is disclosed that patterns to change the incident directions of the ultrasonic wave are stored in advance as a program.
However, the first problem that echo intensity changes in accordance with the tilt angle of each tilted flaw, as described above, is not mentioned in Patent Literature 3 and further, in order to solve the problem, nothing is disclosed about which change pattern should be used to change the incident directions of the ultrasonic wave. In addition, there is a problem similar to the second problem of the method described in Patent Literature 2. That is, there is the problem that the flaw-detection efficiency decreases because electronic scanning must be repeated as many times as the number of tilt angles of the tilted flaws to be detected.
In views of the above-described problems of the related art, the inventors of the present invention have proposed an ultrasonic testing method described in Patent Literature 4 (WO 2007/024000).
More specifically, Patent Literature 4 has proposed an ultrasonic testing method including the steps of: arranging an ultrasonic probe having a plurality of transducers so as to face a tubular test object; and causing transducers appropriately selected from the plurality of transducers to transmit and receive ultrasonic waves so that the ultrasonic waves are propagated in the tubular test object in a plurality of different propagation directions, in which an ultrasonic testing condition by the ultrasonic probe is set so that respective external refraction angles θr of ultrasonic waves in the plurality of the propagation directions are approximately equivalent and/or respective internal refraction angles θk of ultrasonic waves in the plurality of the propagation directions are approximately equivalent (claim 1 and the like of Patent Literature 4).
The ultrasonic probe has the plurality of transducers arranged along an annular curved surface obtained by cutting a predetermined spheroid with two parallel planes facing to each other that do not pass through the center of a spheroid and do not sandwich the center of the spheroid, the two parallel planes being orthogonal to the rotational axis of the spheroid, in the step of arranging the ultrasonic probe so as to face the tubular test object, the ultrasonic probe is arranged so that a longer axis direction of the ultrasonic probe is along an axial direction of the tubular test object, a shorter axis direction of the ultrasonic probe is along a circumferential direction of the tubular test object, and the center of the spheroid correctly faces an axial center of the tubular test object, and a shape of the annular curved surface is determined so that the respective external refraction angles θr of the ultrasonic wave in the plurality of propagation directions are approximately equivalent, and/or the respective internal refraction angles θk of the ultrasonic wave in the plurality of propagation directions are approximately equivalent. (claim 5 and the like of Patent Literature 4).
According to the method described in Patent Literature 4, a plurality of the tilted flaws respectively extending in a direction orthogonal to the plurality of the propagation directions can be detected with high precision. Further, the plurality of flaws can be detected rapidly by transmitting and receiving the ultrasonic waves approximately simultaneously in the plurality of different propagation directions.
Because in-line inspection for inspecting flaws in a sequence of pipe manufacturing processes may be carried out by evaluating to see whether or not there exists any flaw larger than a predetermined dimension rapidly, this can be carried out sufficiently if the ultrasonic testing method proposed by the inventors of the present invention in Patent Literature 4.
On the other hand, a pipe determined to contain flaws in the in-line inspection needs to be inspected again. This reinspection needs to evaluate not only whether or not there exists any flaw but also the position of the flaw in the thickness direction of the pipe (internal surface, external surface, central portion in the thickness direction and the like) and the tilt angle of a tilted flaw in detail by performing flaw detection manually by a qualified inspector.
Although it is demanded upon the aforementioned reinspection that an inspector can evaluate the position and the tilt angle of the flaw easily, Patent Literature 4 has not proposed any solving means for this point. Further, although it is demanded that upon scanning with the ultrasonic probe manually, the posture of the ultrasonic probe with respect to the tubular test object is not changed and a high reliability flaw detection result can be obtained, Patent Literature 4 has not proposed any solving means for this point.